Group iii element nitride crystal producing method and group-iii element nitride crystal

ABSTRACT

A method for producing a high-quality group-III element nitride crystal at a high crystal growth rate, and a group-III element nitride crystal are provided. The method includes the steps of placing a group-III element, an alkali metal, and a seed crystal of group-III element nitride in a crystal growth vessel, pressurizing and heating the crystal growth vessel in an atmosphere of nitrogen-containing gas, and causing the group-III element and nitrogen to react with each other in a melt of the group-III element, the alkali metal and the nitrogen so that a group-III element nitride crystal is grown using the seed crystal as a nucleus. A hydrocarbon having a boiling point higher than the melting point of the alkali metal is added before the pressurization and heating of the crystal growth vessel.

TECHNICAL FIELD

The present invention relates to a group-III element nitride crystalproducing method and a group-III element nitride crystal.

BACKGROUND ART

Group-III element nitride compound semiconductors, such as galliumnitride (GaN) and the like, have attracted attention as materials forsemiconductor devices that emit blue or ultraviolet light. Blue laserdiodes (LDs) are applied to high-density optical discs and displays, andblue light emitting diodes (LEDs) are applied to displays, lights andthe like. Ultraviolet LDs are expected to be applied to biotechnologyand the like, and ultraviolet LEDs are expected to provide ultravioletlight for fluorescent lamps.

A substrate made of a group-III element nitride compound semiconductor,such as gallium nitride (GaN) or the like, for an LD or an LED istypically produced by heteroepitaxially growing a group-III elementnitride crystal on a sapphire substrate using vapor phase epitaxy.Examples of vapor phase epitaxy include Metal Organic Chemical VaporDeposition (MOCVD), Hydride Vapor Phase Epitaxy (HVPE), Molecular BeamEpitaxy (MBE), and the like. However, the dislocation density of agallium nitride crystal obtained by these vapor phase epitaxy methods is10⁸ cm⁻² to 10⁹ cm⁻², which is poor crystal quality. To avoid thisproblem, ELOG (Epitaxial Lateral Overgrowth) has been developed, forexample. This method can reduce the dislocation density to about 10⁵cm⁻² to 10⁶ cm⁻². However, this method disadvantageously includes acomplicated step.

On the other hand, crystal growth may be carried out in liquid phaseinstead of vapor phase epitaxy. Since the nitrogen equilibrium vaporpressure at the melting point of a group-III element nitride singlecrystal, such as gallium nitride (GaN), aluminum nitride (AlN) or thelike, is 10000 atm or more, liquid phase growth of a group-III element,such as a gallium nitride crystal, an aluminum nitride crystal or thelike, needs to be conducted under severe conditions, such as at 1200° C.at 8000 atm (8000×1.013×10⁵ Pa).

To solve this problem, a method of using an alkali metal, such as sodium(Na) or the like, as a flux has been recently developed. This methodallows a group-III element nitride crystal, such as a gallium nitridecrystal, an aluminum nitride crystal or the like, to be obtained underrelatively mild conditions. As an example, in a nitrogen gas atmospherecontaining ammonia, sodium (alkali metal) and gallium (group-IIIelement) are melted by application of pressure and heat, and the melt(sodium flux) is used to conduct crystal growth for 96 hours to obtain agallium nitride crystal having a maximum crystal size of about 1.2 mm(see, for example, Patent Citation 1). Also, a method in which areaction vessel and a crystal growth vessel are separated and a largecrystal is grown while suppressing spontaneous nucleation has beenproposed (see, for example, Patent Citation 2). Also, a method ofgrowing a high-quality bulk crystal, where an alkaline earth metal orthe like is added to sodium, has been proposed (see, for example, PatentCitation 3).

A gallium nitride crystal obtained by the method of using a sodium fluxhas a low disclocation density (i.e., high quality), but a low growthrate (i.e., poor productivity) as compared to when vapor phase epitaxyis employed. Therefore, an improvement in growth rate is required forthe method of producing a gallium nitride crystal in liquid phase, wherean alkali metal, such as sodium or the like, is used as a flux.

-   Patent Citation 1: JP 2002-293696 A-   Patent Citation 2: JP 2003-300798 A-   Patent Citation 3: WO04/013385

DISCLOSURE OF INVENTION

In the method of producing a group-III element nitride crystal using analkali metal as a flux, it is important to efficiently dissolve nitrogenin the flux so as to improve the growth rate. To achieve this, it isnecessary to cause both the temperature of the flux and the pressure ofnitrogen-containing gas (atmosphere gas) to be higher so as to increasethe amount of nitrogen dissolved in flux. However, when the temperatureand the pressure are high, the supersaturation degree of nitrogen in theflux increases near the gas-liquid interface between the atmosphere gasand the flux, so that nonuniform nucleation easily occurs. If nonuniformnucleation occurs at the gas-liquid interface, a polycrystal ofgroup-III element nitride is grown as miscellaneous crystals (so-callednonuniform nucleation) based on the nuclei. Therefore, crystal growththat would otherwise occur on a seed crystal is suppressed,disadvantageously resulting in a reduction in growth rate.

Therefore, an object of the present invention is to provide a method forproducing a group-III element nitride crystal, in which a crystal growthrate can be improved by suppressing the occurrence of miscellaneouscrystals.

To achieve the object, a method for producing a group-III elementnitride crystal according to the present invention includes the steps ofadding a hydrocarbon having a boiling point higher than the meltingpoint of an alkali metal to a crystal growth vessel containing agroup-III element, an alkali metal, and a seed crystal of group-IIIelement nitride, pressurizing and heating the crystal growth vessel inan atmosphere of nitrogen-containing gas, and causing the group-IIIelement and nitrogen to react with each other in a melt of the group-IIIelement, the alkali metal and the nitrogen so that a group-III elementnitride crystal is grown using the seed crystal as a nucleus.

Also, a group-III element nitride crystal according to the presentinvention is one that has an optical absorption coefficient of 10 cm⁻¹or less with respect to light having a wavelength of 400 nm or more and620 nm or less and is produced by the above-described method.

A semiconductor device formation substrate according to the presentinvention includes the group-III element nitride crystal of the presentinvention.

Moreover, a semiconductor device according to the present invention issuch that a semiconductor layer is formed on the substrate of thepresent invention.

By adding a hydrocarbon having a specific boiling point to the fluxduring the growth of a group-III element nitride crystal using an alkalimetal flux, nonuniform nucleation can be suppressed. Thereby, it ispossible to efficiently grow a group-III element nitride crystal athigher temperatures and higher pressures. As a result, the crystalgrowth rate can be improved. Also, by coating the alkali metal with ahydrocarbon, it is possible to suppress reaction of the alkali metalwith oxygen and water in an atmosphere. Further, by adding ahydrocarbon, the film thickness of a grown crystal can be uniform. As aresult, the quality of the group-III element nitride crystal can beimproved as well. The production method of the present invention iseffective to general group-III element nitride crystals, and isparticularly effective when the alkali metal is sodium and the group-IIIelement nitride crystal is a gallium nitride crystal.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1( a) is a diagram showing an exemplary configuration of aproduction apparatus for use in a production method according to thepresent invention, and FIG. 1( b) is a diagram showing an exemplaryclosed pressure-resistant and heat-resistant vessel for use in theproduction method of the present invention.

FIG. 2( a) shows a gallium nitride crystal for production number 3 ofExample 1, and FIG. 2( b) shows a gallium nitride crystal for productionnumber 5 of Example 1.

FIG. 3( a) shows a gallium nitride crystal for production number 9 ofExample 2, and FIG. 3( b) shows a gallium nitride crystal for productionnumber 11 of Example 2.

FIG. 4 shows a gallium nitride crystal for production number 12 ofExample 3.

FIG. 5 is a diagram showing an exemplary configuration of a productionapparatus used in Example 4.

FIGS. 6( a) to 6(c) are schematic diagrams showing exemplary states of asubstrate and a crystal of Example 4.

FIGS. 7( a) to 7(c) are schematic diagrams showing exemplary states of asubstrate and a crystal of Example 5.

EXPLANATION OF REFERENCE

-   1 gas supply apparatus-   2 pipe-   3 pressure adjuster-   4 pipe-   5 valve-   7 joint-   9 valve-   10 heat-resistant pipe-   11 pipe-   12 pipe-   13 valve-   14 exhaust apparatus-   15 closed pressure-resistant and heat-resistant vessel-   16 heating apparatus-   17 reaction vessel-   18 crystal growth vessel-   20 seed crystal-   21 flux-   30 gallium nitride crystal-   31 miscellaneous crystals-   32 gallium nitride crystal-   60 high-pressure chamber-   61 high-pressure chamber lid-   62 gas flow rate adjuster-   64 joint-   65 gas inlet-side valve-   66 gas outlet-side valve-   68 pressure adjuster-   70 heater-   72 heat insulator-   80 reaction vessel-   82 crystal growth vessel-   84 sodium-   86 gallium-   88 seed-   100 holding substrate-   102 seed layer-   104 grown crystal-   106 self-sustaining substrate-   108 grown crystal-   110 self-sustaining substrate

BEST MODE FOR CARRYING OUT THE INVENTION

A method for producing a group-III element nitride crystal according tothe present invention is performed as follows. A group-III element, analkali metal, and a seed crystal of group-III element nitride are placedin a crystal growth vessel, to which a hydrocarbon having a boilingpoint higher than the melting point of the alkali metal is then added.The crystal growth vessel is pressurized and heated in anitrogen-containing gas atmosphere so that the group-III element andnitrogen are caused to react with each other in the melt containing thegroup-III element, the alkali metal and nitrogen. As a result, agroup-III element nitride crystal is grown on the seed crystal as anucleus. In particular, the alkali metal is preferably coated with ahydrocarbon, and the alkali metal coated with the hydrocarbon ispreferably added to the crystal growth vessel.

In the present invention, the group-III element is gallium (Ga),aluminum (Al) or indium (In), particularly preferably gallium (Ga). Thegroup-III element nitride is, for example, aluminum nitride (AlN),indium nitride (InN), gallium nitride (GaN) or the like, particularlydesirably gallium nitride (GaN). The alkali metal is lithium (Li),sodium (Na), potassium (K), rubidium (Rb), cesium (Cs) or francium (Fr),particularly desirably sodium (Na). These alkali metals may be usedsingly or in combination of two or more. In this case, the major alkalimetal component is desirably sodium. As a flux component, an alkalineearth metal may be used. The alkaline earth metal is, for example, Mg,Ca, Sr or Ba. A dopant may also be added to the flux. An n-type dopantis, for example, Si, Ge, Sn or O. A p-type dopant is, for example, Mg,Ca, Sr, Ba or Zn. The amount of the dopant in the obtained crystal fallswithin the range of 1×10¹⁵ to 1×10¹⁹ cm⁻³, for example.

Hereinafter, an embodiment of the production method of the presentinvention will be described, assuming that the alkali metal is sodium(Na), the group-III element is gallium (Ga), and the group-III elementnitride crystal is gallium nitride (GaN). Note that a group-III elementnitride crystal other than gallium nitride may also be used and may beproduced with reference to the following description.

Embodiment 1

In this embodiment, a method for producing a gallium nitride crystalwill be described in which a seed crystal, sodium, gallium, and ahydrocarbon are placed in a crystal growth vessel for holding a sodiumflux, and thereafter, nitrogen-containing gas is pressurized and thecrystal growth vessel is heated to melt sodium and gallium, therebygenerating a sodium flux.

An exemplary configuration of a production apparatus that is employed inthe production method of the present invention is shown in FIG. 1( a).An exemplary closed pressure-resistant and heat-resistant vessel for usein the production method of the present invention is shown in FIG. 1(b).

The apparatus of FIG. 1( a) comprises a gas supply apparatus 1 forsupplying material gas, a pressure adjuster 3 for adjusting the pressureof the material gas, a closed pressure-resistant and heat-resistantvessel 15 for conducting crystal growth, a heating apparatus 16 forheating, and an exhaust apparatus 14.

The gas supply apparatus 1, which is filled with nitrogen-containing gasas the material gas, is connected via a pipe 2 to a pressure adjuster 3.

The pressure adjuster 3, which has a function of adjusting the materialgas into an optimal gas pressure, is connected via a pipe 4 and a valve5 to a detachable joint 7. Also, a portion of the pipe 4 (portionindicated with a wavy line) is formed of a pressure-resistant flexiblehose, thereby enabling it to freely change the position and thedirection of the joint 7. Moreover, the pipe 4 branches into a pipe 11at an intermediate portion thereof. The pipe 11 is connected via a valve13 and a pipe 12 to the exhaust apparatus 14.

On the other hand, a reaction vessel 17 having a valve 9, aheat-resistant pipe 10 and the closed pressure-resistant andheat-resistant vessel 15 is connected to the joint 7. Note that thereaction vessel 17 is detachably connected to the joint 7.

As the heating apparatus 16, an electric furnace including a heatinsulator and a heater can be used, for example. Also, the heatingapparatus 16 preferably performs a temperature control so that thetemperature of the closed pressure-resistant and heat-resistant vessel15 and a portion within the heating apparatus 16 of the heat-resistantpipe 10 is maintained uniform, particularly in terms of prevention ofaggregation of the sodium flux. Temperature the heating apparatus 16 canbe controlled to, for example, 600° C. (873 K) to 1100° C. (1373 K). Thepressure adjuster 3 can control the nitrogen-containing gas within therange of 100 atm (10×1.01325×10⁵ Pa) or less. The heating apparatus 16also has a shaking function. The closed pressure-resistant andheat-resistant vessel 15 can be fixed to the heating apparatus 16 so asto be shaken.

FIG. 1( b) shows a configuration of the closed pressure-resistant andheat-resistant vessel 15. A crystal growth vessel 18 is provided in theclosed pressure-resistant and heat-resistant vessel 15. A seed crystal20 is vertically arranged in the crystal growth vessel 18. The crystalgrowth vessel 18 is filled with a flux 21 of melted gallium (Ga) andsodium (Na). The seed crystal 20 may be horizontally arranged on abottom of the crystal growth vessel 18 instead of the verticalarrangement as shown in the figure. Note that, in the case of the bottomarrangement, a surface of the seed crystal 20 on which crystal growth isconducted needs to be caused to face upward.

Examples of a material for the crystal growth vessel 18 include, but arenot particularly limited to, alumina (Al₂O₃), yttria (Y₂O₃), BN, PBN,MgO, CaO, W, SiC, carbon materials (graphite, diamond-like carbon,etc.), and the like. Particularly, yttria or alumina, which hindersdissolution of oxygen and aluminum into the flux even at hightemperatures, preferably enables growth of a gallium nitride crystalincluding less impurities.

Examples of a material for the closed pressure-resistant andheat-resistant vessel 15 include SUS materials (SUS316, etc.), nickelalloys (Inconel, Hastelloy, Incoloy, etc.), and the like, which areresistant to high temperatures. In particular, materials, such asInconel, Hastelloy, Incoloy and the like, are resistant to oxidation athigh temperatures and high pressures, and can also be used inatmospheres other than inert gas, and are preferable in terms ofreusability and durability.

Next, production of a gallium nitride crystal using the productionapparatus described above will be described.

Initially, sodium and gallium as materials and the seed crystal 20 as anucleus (template) for crystal growth are placed in the crystal growthvessel 18.

In the present invention, sodium as a material is desirably of highpurity in terms of the suppression of nonuniform nucleation and the highquality of a crystal. Specifically, the purity is 99% or more, morepreferably 99.95% or more. The purity of gallium is similarly preferably99% or more, more preferably 99.9% or more. The mass ratio (Na:Ga) ofsodium (Na) and gallium (Ga) is preferably within the range of Na:Ga=4:1to 1:4, more preferably Na:Ga=2:1 to 1:2, taking the solubility ofnitrogen into consideration. The total amount of sodium and gallium isset such that the range of the depth of the seed crystal 20 from aliquid surface of the flux 21 in the crystal growth vessel 18 has apredetermined value. The range of the depth from the liquid surface ofthe flux 21 to the seed crystal 20 is preferably 0 mm to 40 mm, morepreferably 2 mm to 20 mm.

In the present invention, the seed crystal may be any of a singlecrystal, a polycrystal, and an amorphous substance, though a singlecrystal or an amorphous substance is desirable. The form of the nucleusis not particularly limited and is desirably a single-crystal substrateof gallium nitride or a thin film substrate of gallium nitride, forexample. The thin film substrate of gallium nitride is provided byforming a gallium nitride thin film on a substrate made of sapphireusing, for example, Metal Organic Chemical Vapor Deposition (MOCVD),Hydride Vapor Phase Epitaxy (HVPE), Molecular Beam Epitaxy (MBE), or thelike. The gallium nitride thin film preferably has a thickness of 5 μmor more, more preferably 10 μm or more. This is because when thetemperature of the flux exceeds 700° C. as temperature increases, theamount of nitrogen that can be dissolved rapidly increases and surpassesthe supply of nitrogen from the atmosphere gas. As a result, there is anextreme shortage of nitrogen in the flux, so that gallium nitride of theseed crystal is extremely dissolved (excessive meltback). Therefore, thegallium nitride thin film preferably has a large thickness.

Next, a hydrocarbon is added to the crystal growth vessel 18. Thehydrocarbon is preferably liquid, solid or a mixture of liquid and solidat room temperature (e.g., 25° C.). In the present invention, thehydrocarbon is preferably, for example, a chain saturated hydrocarbon, achain unsaturated hydrocarbon, an alicyclic hydrocarbon, an aromatichydrocarbon or the like, or a mixture thereof. The hydrocarbon alsopreferably does not contain oxygen, so as to prevent oxidation of analkali metal. Note that since the hydrocarbon evaporates in the step ofheating the crystal growth vessel 18 to generate a flux, the hydrocarbonpreferably has a high boiling point. The boiling point of thehydrocarbon is suitably higher than or equal to the boiling point of thealkali metal. As used herein, “the boiling point of the hydrocarbon ishigher than or equal to the melting point of the alkali metal” meansthat when there are two or more alkali metals, the boiling point of thehydrocarbon is higher than or equal to the melting point of at least oneof the two or more alkali metals.

For example, the boiling point of the hydrocarbon is higher than orequal to 97.7° C., which is the melting point of sodium, preferably 150°C. or more, and more preferably 300° C. or more. In the presentinvention, the hydrocarbon is, for example, kerosenes having a boilingpoint of 150° C. or more, paraffins having a boiling point of 300° C. ormore (e.g., heptadecane (boiling point: 302° C.), octadecane (boilingpoint: 317° C.), nanodecane (boiling point: 330° C.), icosane (boilingpoint: 342° C.), triacontane (boiling point: 449.8° C.), vaseline(boiling point: 302° C.), liquid paraffins (boiling point: 170 to 340°C.), and solid paraffins (boiling point: 300° C. or more)), lamp oilshaving a boiling point of 150 to 250° C., biphenyl (boiling point: 254°C.), o-xylene (boiling point: 144° C.), m-xylene (boiling point: 139°C.), p-xylene (boiling point: 138° C.), cumene (boiling point: 153° C.),ethyltoluenes (boiling point: 161 to 165° C.), cumene (boiling point:177° C.), tetralin (boiling point: 208° C.), or the like. These may beused singly or in combination of two or more. Of these hydrocarbons,octadecane, nanodecane, icosane, triacontane, solid paraffin andbiphenyl are solid at room temperature. Heptadecane having a boilingpoint of 300° C. or more, liquid paraffins, kerosines having a boilingpoint of 150° C. or more, lamp oils having a boiling point of 150 to250° C., o-xylene, m-xylene, p-xylene, cumene, ethyltoluene, cymene, andtetralin are liquid at room temperature. Of these hydrocarbons, liquidparaffins are preferable, which have a low vapor pressure at roomtemperature and are liquid and therefore easy to weigh.

The lower limit of the amount of the hydrocarbon added is preferably0.03 parts by mass (%) or more, more preferably 0.05 parts by mass (%)or more, per 100 parts by mass of an alkali metal (e.g., sodium).

The upper limit of the amount of the hydrocarbon added is preferablydetermined as appropriate, when a single crystal of gallium nitride isused as a seed crystal and when a thin film substrate of gallium nitrideis used.

When a single crystal of gallium nitride is used as a seed crystal, theamount of the hydrocarbon added is preferably, but is not particularlylimited to, one part by mass (%) or less per 100 parts by mass ofsodium. If the amount of the hydrocarbon added exceeds 1%, a reductionin quality, such as an increase in defect or coloration of galliumnitride crystal or the like, is likely to occur.

On the other hand, when a thin film substrate is used as a seed crystal(e.g., a thin film substrate made of gallium nitride having a thicknessof about 10 μm is used, the upper limit of the amount of the hydrocarbonadded is preferably 0.6 parts by mass (%) or less, more preferably 0.4parts by mass (%) or less, per 100 parts by mass of an alkali metal(e.g., sodium). This is because the hydrocarbon promotes meltback. Notethat “meltback” means that when the temperature of the sodium fluxincreases during an initial period of crystal growth, the amount ofnitrogen that can be dissolved in the sodium flux rapidly increases,whereas the dissolution of nitrogen into the sodium flux slowed down, sothat gallium nitride is dissolved from the seed crystal. If the amountof dissolved nitrogen is thus large, then when, for example, a thin filmcrystal is used as a seed crystal, the seed may be dissolved and lost(excessive dissolution or excessive meltback). Therefore, when a thinfilm substrate of gallium nitride is used as a seed crystal, the amountof the hydrocarbon is preferably set to fall within the range describedabove. Note that when the gallium nitride thin film has a thickness ofmore than 10 μm, the upper limit of the amount of the hydrocarbon addedis preferably increased in substantially proportion to the thickness ofthe thin film. When the gallium nitride has a thickness of less than 10μm, the upper limit is preferably decreased in proportion to thethickness.

Next, the crystal growth vessel 18 is placed in the closedpressure-resistant and heat-resistant vessel 15, and then the closedpressure-resistant and heat-resistant vessel 15, the valve 9 and thepipe 10 are assembled into the reaction vessel 17, which is thenconnected to the joint 7. Thereafter, the valve 9 is closed so as toprevent entry of atmosphere gas. These preliminary steps are preferablyconducted in an atmosphere of inert gas (e.g., nitrogen gas, argon,etc.) whose oxygen concentration and water content are controlled, so asto suppress oxidation and hydroxylation of sodium. The oxygenconcentration of the inert gas is preferably 5 ppm or less, morepreferably 1 ppm or less. The water content (by volume) of the inert gasis preferably 3 ppm or less, more preferably 0.5 ppm or less.Particularly, more preferably, the gas atmosphere has an oxygenconcentration of 1 ppm or less and a water content (by volume) of 0.5ppm or less. In this case, oxidation and hydroxylation of sodium surfacecan be hindered for several hours.

Next, the reaction vessel 17 is removed from the inert gas atmosphere,the closed pressure-resistant and heat-resistant vessel 15 is placed inthe heating apparatus 16, and the joint 7 is connected to the pipe 4.Thereafter, the valve 13 is opened, and the exhaust apparatus 14 is usedto remove gas from the closed pressure-resistant and heat-resistantvessel 15 through the pipe 4. After the end of the gas removal, thevalve 13 is closed and the valve 5 and the valve 9 are opened, so thatthe inside of the closed pressure-resistant and heat-resistant vessel 15is pressurized with nitrogen-containing gas from the gas supplyapparatus 1. Note that the applied pressure is adjusted by the pressureadjuster 3.

Next, the closed pressure-resistant and heat-resistant vessel 15 isheated by the heating apparatus 16 to melt sodium and gallium in thecrystal growth vessel 18, thereby generating a sodium flux. Thereafter,after a lapse of about 10 to 30 hours, dissolution of nitrogen in thesodium flux reaches supersaturation. Thereby, gallium nitride crystal isdeposited on the seed crystal 20. Further, by continuing heating andpressurization for a predetermined time, gallium nitride crystal isfurther grown. When about 70% to 95% of gallium supplied as a materialhas been deposited as gallium nitride crystal, the seed crystal 20 isremoved from the crystal growth vessel 18.

Here, the heating conditions are appropriately determined, depending ona component of the flux and a component of the atmosphere gas and itspressure. For example, the heating temperature is within the range of700° C. (973 K) to 1100° C. (1373 K), preferably the range of 800° C.(1073 K) to 1000° C. (1273 K). For example, the pressurizationconditions are 2 atm (2×1.01325×10⁵ Pa) or more, preferably 20 atm(20×1.01325×10⁵ Pa) or more. The upper limit of the pressurizationconditions is preferably 100 atm (100×1.01325×10⁵ Pa) or less.

The nitrogen-containing gas is, for example, nitrogen gas (N₂), ammoniagas (NH₃) or the like, which may or may not be mixed (the mixture ratiois not limited). In particular, the use of ammonia gas is preferablesince a reaction pressure can be reduced. Also, the nitrogen-containinggas preferably has a low oxygen concentration and a low water content soas to suppress oxidation and hydroxylation of the flux. The oxygenconcentration of the nitrogen-containing gas is preferably 5 ppm orless, more preferably 1 ppm or less. The water content (by volume) ofthe nitrogen-containing gas is preferably 3 ppm or less, more preferably0.1 ppm or less.

Note that the principle of the effect of suppressing nonuniformnucleation by addition of a hydrocarbon is as follows. The hydrocarbonis broken down to carbon and hydrogen in the flux. Carbon in the flux iseasily coupled with nitrogen to form a cyanide ion. Therefore, theamount of nitrogen that can be dissolved in the flux increases, andnitrogen is diffused due to convection in the flux near the gas-liquidinterface without reaching excessive supersaturation. On the other hand,hydrogen in the flux acts to break down gallium nitride at hightemperatures as is similar to when it is in gas. Therefore, hydrogeninvariably breaks down the seed crystal of gallium nitride or nucleatedgallium nitride. In particular, a small gallium nitride immediatelyafter nucleation has an excessively large surface area as compared toits volume, and therefore, is more effectively broken down orextinguished. Thus, carbon and hydrogen allow relaxation of excessivesupersaturation near the gas-liquid interface of the flux or the like.Moreover, due to hydrogen, gallium nitride immediately after nucleationcannot grow and is broken down. It can be considered that, as theirsynergetic effect, the occurrence of miscellaneous crystals near thegas-liquid interface can be suppressed, and in addition, the filmthickness of gallium nitride grown on the seed crystal is caused to bemore uniform.

Embodiment 2

In this embodiment, a method for producing gallium nitride will bedescribed in which a seed crystal, sodium (alkali metal) coated with ahydrocarbon, gallium (group-III element), and a hydrocarbon are placedin a crystal growth vessel for holding a sodium flux that is an alkalimetal, and thereafter, a portion of the hydrocarbon is removed, followedby heating of the crystal growth vessel.

In this production method, further, the amount of the hydrocarbon in thecrystal growth vessel is preferably adjusted. For example, thesuppression of nucleation and the coating of the alkali metal requiredifferent amounts of the hydrocarbon. Therefore, the hydrocarbon may beadded in an amount required for the coating of the alkali metal in apreliminary step, and the amount of the hydrocarbon may be adjustedwithin an appropriate range so as to prevent dissolution of the seedcrystal in an adjustment step. The adjustment of the amount of thehydrocarbon includes, for example, removal or addition of thehydrocarbon.

An exemplary production apparatus for use in the production method ofthe present invention is similar to that of FIGS. 1( a) and 1(b)described in Embodiment 1. A procedure for producing a gallium nitridecrystal using this production apparatus will be described.

Initially, sodium and gallium as materials and a seed crystal as anucleus (template) for crystal growth are placed in the crystal growthvessel 18. Here, sodium that is coated in advance with a hydrocarbon isused. Sodium is coated with a hydrocarbon as follows. For example,sodium from which oxidized and hydroxylated portions thereof have beenremoved may be immersed in a liquid hydrocarbon and then pulled out.Alternatively, a hydrocarbon that is solid at room temperature may beheated in advance to be melted, and then sodium may be immersed in themelt and then pulled out, and the temperature of the hydrocarbon may bethen returned to room temperature so that the hydrocarbon is solidified.Also, a liquid hydrocarbon and a solid hydrocarbon may be mixed.

In the present invention, the boiling point of a hydrocarbon used forcoating is preferably sufficiently higher than room temperature so asmaintain the effect of preventing oxidation and hydroxylation of sodiumat room temperature in addition to the reason of Embodiment 1.Specifically, the boiling point of the hydrocarbon is higher than orequal to 97.7° C. that is the melting point of sodium, more preferably150° C. or more, and even more preferably 300° C. or more. Examples ofthe hydrocarbon include hydrocarbons similar to those described inEmbodiment 1.

If the mass of the hydrocarbon for coating sodium is excessively large,a reduction in quality, such as an increase in defect, coloration ofgallium nitride crystal or the like, is likely to occur. As a guideline,one part by mass (%) or less of the hydrocarbon is preferably added per100 parts by mass of sodium. When the seed crystal is a thin filmsubstrate made of gallium nitride having a thickness of about 10 μm, theamount of the hydrocarbon is preferably 0.6 parts by mass (%) or less,more preferably 0.4 parts by mass (%) or less, per 100 parts by mass ofsodium. However, the mass of the hydrocarbon for coating sodium mayoften exceed the above-described upper limit, depending on the shape ofsodium or the thickness of the coating film of the hydrocarbon. In thiscase, a portion of the hydrocarbon is preferably removed in anadjustment step described below so as to obtain an appropriate amount ofthe hydrocarbon to be left in the crystal growth vessel 18.

Next, the crystal growth vessel 18 is placed in the closedpressure-resistant and heat-resistant vessel 15, and then the closedpressure-resistant and heat-resistant vessel 15, the valve 9 and thepipe 10 are assembled into the reaction vessel 17, which is thenconnected to the joint 7. Thereafter, the valve 9 is closed so as toprevent entry of atmosphere gas. These preliminary steps are preferablyconducted in an atmosphere of inert gas (e.g., nitrogen gas, argon,etc.) whose oxygen concentration and water content are controlled, so asto suppress oxidation and hydroxylation of sodium. When sodium coatedwith a hydrocarbon is used, the gas atmosphere preferably has an oxygenconcentration of 10 ppm or less and a water content (by volume) of 10ppm or less. This is because oxidation and hydroxylation can be hinderedfor several hours.

Next, the reaction vessel 17 is removed from the inert gas atmosphere,the closed pressure-resistant and heat-resistant vessel 15 is placed inthe heating apparatus 16, and the joint 7 is connected to the pipe 4.Thereafter, the valve 13 is opened, and the exhaust apparatus 14 is usedto remove gas from the closed pressure-resistant and heat-resistantvessel 15 through the pipe 4, and optionally, perform an adjustment stepof removing a portion of the hydrocarbon coating the sodium. The removalof a portion of the hydrocarbon to obtain an appropriate amount ofremainder thereof may be, for example, carried out by adjusting the timelength of the gas removal (exhaust time length). Since the exhaust timelength varies from hydrocarbon to hydrocarbon, the exhaust time lengthis desirably determined by previously measuring the exhaust time lengthand the remaining amount of the hydrocarbon in the crystal growth vessel18, or may be desirably determined based on the state of meltback or thestate of a crystal after growth of a gallium nitride crystal. Thehydrocarbon that coats the sodium may be solid or a mixture of solid andliquid at room temperature. When the vapor pressure is about 1000 Pa orLess, the exhaust time may be considerably long. In this case, theclosed pressure-resistant and heat-resistant vessel 15 is desirablyheated while the gas is being removed. The heating temperature ispreferably higher than room temperature and lower than 700° C. that isthe crystal growth temperature. In view of the removal of water or thelike attached by the time the reaction vessel 17 is assembled, theheating temperature is preferably higher than or equal to 100° C. andlower than or equal to 490° C. that is a temperature at which sodium andgallium form an alloy. Alternatively, a portion of the hydrocarboncoating the sodium can be removed, for example, by performing pressureapplication and pressurized cleaning purge with respect to the closedpressure-resistant and heat-resistant vessel 15 a predetermined numberof times, or by providing an exhaust outlet (not shown) in the closedpressure-resistant and heat-resistant vessel 15 and causing inert gas,such as nitrogen gas or the like, to flow near the crystal growth vesselfor a predetermined time. These methods can also be combined withheating of the closed pressure-resistant and heat-resistant vessel 15.

Next, the inside of the closed pressure-resistant and heat-resistantvessel 15 is pressurized with nitrogen-containing gas from the gassupply apparatus 1. Further, the closed pressure-resistant andheat-resistant vessel 15 is heated to a crystal growth temperature bythe heating apparatus 16, thereby growing a gallium nitride crystal.Thereafter, when about 70% to 95% of gallium supplied as a material hasbeen deposited as gallium nitride crystal, the seed crystal 20 isremoved from the crystal growth vessel 18. In this case, gallium nitridecrystal has been grown on the seed crystal 20.

Embodiment 3

A group-III element nitride crystal of the present invention is producedby the production method of the present invention described above.

This crystal preferably has an optical absorption coefficient of 10 cm⁻¹or less with respect to light having a wavelength of 400 nm or more and620 nm or less. The optical absorption coefficient is preferably 5 cm⁻¹or less. Note that the lower limit of the optical absorption coefficientis a value exceeding zero.

The group-III element nitride crystal produced by the production methodof the present invention may contain carbon. For example, the group-IIIelement nitride crystal of the present invention may contain 5×10¹⁷(cm⁻³) or less carbon atoms as a result of analysis by SIMS or the like.

In such a crystal, the group-III element is at least one elementselected from Al, Ga and In. The group-III element nitride is preferablya compound that is represented by Al_(s)Ga_(t)In_((1-s-t))N, where0≦s≦1, 0≦t≦1, and s+t≦1.

In particular, the group-III element is preferably gallium (Ga), and thegroup-III element nitride is preferably gallium nitride (GaN).

Embodiment 4

A substrate of the present invention includes a group-III elementnitride crystal produced by the production method of the presentinvention described above.

A semiconductor device of the present invention is a semiconductordevice in which a semiconductor layer is formed on the substrate.

The semiconductor layer is not particularly limited and may be anycompound semiconductor, such as Al_(s)Ga_(t)In_((1-s-t))N or the like.The semiconductor layer may have either a single-layer structure or amultilayer structure.

Examples of a semiconductor device formed using such a substrate includea laser diode (LD), a light emitting diode (LED) and the like.

Hereinafter, examples of the present invention will be described.

Example 1

Gallium nitride crystals were produced using the apparatus of FIG. 1under six sets of conditions in accordance with the description ofEmbodiment 1 above. Generation of the gallium nitride crystals wasconfirmed and the effect of suppressing nonuniform nucleation byaddition of hydrocarbons was confirmed. Hereinafter, the productioncondition will be described.

(Production Conditions)

Seed crystal: gallium nitride thin film substrate

Dimension: 14 mm×15 mm gallium nitride thin film (film thickness: 10 μm)

Gas species: nitrogen gas (N₂), purity 5 N

Sodium: purity 99.9 to 99.99%

Gallium: purity 99.999 to 99.99999%

Hydrocarbons:

-   -   Liquid paraffin, specific gravity 0.82, boiling point: 170 to        340° C.    -   Solid paraffin, specific gravity 0.90, boiling point: 300° C. or        more    -   Lamp oil, specific gravity 0.80, boiling point: 170° C. to 250°        C.

Crucible used: Al₂O₃ (alumina), purity 99.9 to 99.99%

Growth time: 144 hours

Direction in which seed crystal is placed: vertical

Table 1 below shows production conditions, i.e., the mass of sodium(Na), the mass of gallium (Ga), the type and mass of a hydrocarbon (HC),and a pressure and a temperature during growth. Note that productionnumbers 1 to 3 indicate comparative examples and production numbers 4 to6 indicate Example 1. The results of production under the six sets ofconditions are shown in Table 2 below. Note that measurement andevaluation in Table 2 were carried out by a method described below.

Generation of gallium nitride was confirmed by conducting elementanalysis (EDX) and photoluminescence measurement (PL). Element analysiswas conducted by electron irradiation having an acceleration voltage of15 kV while confirming the position of a sample using an electronmicroscope. A photoluminescence measurement was conducted byhelium-cadmium laser irradiation at room temperature.

The amount of gallium nitride was measured by separately measuring theamount of a grown crystal and the amount of miscellaneous crystals grownusing a method below. The crystal growth amount was obtained bysubtracting the mass of a crystal seed alone previously measured fromthe mass of a seed crystal (including a crystal growth portion) aftercrystal growth. The amount of miscellaneous crystals was obtained bycollecting crystals attached to the inner surface of the crystal growthvessel (crucible) and measuring the mass of the collected crystals.

The yield of gallium nitride (GaN) was obtained as the proportion of amass corresponding to gallium of an amount (=crystal growth amount)grown on the seed crystal substrate with respect to the mass of galliumoriginally supplied.

An upper-lower film thickness ratio was obtained as a lower thickness/anupper thickness, where the upper thickness is the thickness of crystalgrowth at a portion located 5 mm from the upper end of the verticallyarranged seed crystal and the lower thickness is the thickness ofcrystal growth at a portion located 5 mm from the lower end thereof.

TABLE 1 Growth Growth Production Na Ga Hydrocarbon HC/Na temperaturepressure No. (g) (g) (HC) (mg) (wt %) (° C.) (MPa) 1 2.3 2.0 None — 0850 3.4 2 2.3 2.0 None — 0 860 3.6 3 2.3 2.0 None — 0 870 3.8 4 2.3 2.0Lamp 7.9 0.34 870 3.8 oil 5 2.3 2.0 Liquid 8.1 0.35 870 3.8 paraffin 62.3 2.0 Solid 9.0 0.39 870 3.8 paraffin

TABLE 2 Upper-lower Crystal Miscellaneous film Produc- Confirmationgrowth crystals GaN thickness tion of amount growth yield ratio No.generation (g) amount (g) (%) (lower/upper) 1 Confirmed 1.30 0.78 54 0.32 Confirmed 0.72 1.30 30 0.3 3 Confirmed 0.56 1.39 27 0.4 4 Confirmed1.72 0.17 72 0.8 5 Confirmed 2.06 0.00 87 1.1 6 Confirmed 2.02 0.00 841.2

As shown in Table 2, for all production numbers 1 to 6, generation ofgallium nitride was confirmed. For production numbers 1 to 3 in which nohydrocarbon was added, a large amount of miscellaneous crystals thatwere considered to be caused by nonuniform nucleation were grown in thecrucible. Particularly, when the temperature and the pressure are high,the amount of miscellaneous crystals grown increases and the amount ofcrystal growth decreases. At the same time, the yield of gallium nitridedecreases. On the other hand, for production numbers 4 to 6 in which ahydrocarbon was added, substantially no miscellaneous crystals weregenerated, i.e., the effect of suppressing nonuniform nucleation byaddition of the hydrocarbon was confirmed. At the same time, the valueof the upper-lower film thickness ratio was improved, and the effect ofgrowth with more uniform film thicknesses was confirmed.

FIGS. 2( a) and 2(b) show photographs of gallium nitride crystals 30 ofproduction numbers 3 and 5. As shown in FIG. 2( a), when no hydrocarbonwas added, a considerably large amount of miscellaneous crystals 31 wasgenerated. On the other hand, as shown in FIG. 2( b), when a hydrocarbon(lamp oil) was added, generation of the miscellaneous crystals 31 wassuppressed.

Example 2

Gallium itride crystals were produced under five sets of conditions(production numbers 7 to 11) using the apparatus of FIG. 1 in a mannersimilar to that of Embodiment 1 described above, where a gallium nitridethin film was used as a seed crystal. Hereinafter, the productionconditions will be described.

(Production Conditions)

Seed crystal: gallium nitride thin film substrate

Dimension: 14 mm×15 mm gallium nitride thin film (film thickness: 10 μm)

Gas species: nitrogen gas (N₂), purity 99.999%

Sodium: purity 99.9 to 99.99%

Gallium: purity 99.999 to 99.99999%

Hydrocarbon: Liquid paraffin, specific gravity 0.82, boiling point: 170to 340° C.

Crucible used: Al₂O₃ (alumina), purity 99.9 to 99.99%

Growth time: 144 hours

Direction in which seed crystal is placed: vertical

Table 3 described below shows production conditions, i.e., the mass ofsodium (Na), the mass of gallium (Ga), the type and mass of ahydrocarbon (HC), and a pressure and a temperature during growth. Theresults of production under the five sets of conditions are also shownin Table 3 below. Here, confirmation of generation of gallium nitride,the amount of a grown crystal, the amount of miscellaneous crystalsgrown, and the yield of gallium were measured by a method similar tothat of Example 1. A growth area ratio indicates the proportion of agrown gallium nitride crystal with respect to the area of a galliumnitride thin film as a seed crystal.

TABLE 3 Growth Growth Production Na Ga Hydrocarbon HC/Na temperaturepressure No. (g) (g) (HC) (mg) (wt %) (° C.) (MPa) 7 2.3 2.0 Liquid 0.410.018 865 3.6 paraffin 8 2.3 2.0 Liquid 0.82 0.036 865 3.6 paraffin 92.3 2.0 Liquid 4.06 0.18 865 3.6 paraffin 10 2.3 2.0 Liquid 8.12 0.35865 3.6 paraffin 11 2.3 2.0 Liquid 16.2 0.70 865 3.6 paraffin

TABLE 4 Crystal Miscellaneous Growth Produc- Confirmation growthcrystals GaN area tion of amount growth yield ratio No. generation (g)amount (g) (%) (%) 7 Confirmed 1.64 0.23 68 100 8 Confirmed 1.99 0.02 8398 9 Confirmed 2.00 0.00 84 98 10 Confirmed 1.63 0.02 70 95 11 Confirmed0.59 0.00 24 53

Initially, it was confirmed that a gallium nitride crystal was generatedunder all the sets of conditions (i.e., production numbers 7 to 11).Whereas the effect of suppressing generation of miscellaneous crystalsby addition of a hydrocarbon was confirmed for all production numbers 7to 11, generation of miscellaneous crystals was more effectivelysuppressed for production numbers 8 to 11 in which a larger amount of ahydrocarbon was added. According to these results, the amount of ahydrocarbon added is more preferably about 0.03% or more by mass withrespect to the amount of sodium in terms of the effect of suppressingthe occurrence of miscellaneous crystals. On the other hand, asatisfactory growth area ratio was obtained when the amount of ahydrocarbon added was less than 0.7 wt %, and therefore, it can be saidthat the melting during an initial state of crystal growth of a nitrogengallium thin film (excessive meltback) was suppressed. If the amount ofa hydrocarbon added was 0.35 wt % or less, the growth area ratio was 95%or more, and the gallium nitride thin film was substantially not melted.According to these results, when a gallium nitride thin film is used asa seed crystal, the appropriate range of the amount of a hydrocarbonadded is more preferably 0.4% or less by mass with respect to the amountof sodium in terms of prevention of excessive meltback.

FIGS. 3( a) and 3(b) show photographs of gallium nitride crystals 32 forproduction numbers 9 and 11. FIG. 3( a) shows a case where a hydrocarbonfor production number 9 was added in an amount of 0.18 wt %. In thiscase, gallium nitride was grown on an entire surface of the seedsubstrate, and there was substantially no excessive meltback of thegallium nitride thin film. FIG. 3( b) shows a case where a hydrocarbonfor production number 11 was added in an amount of 0.7 wt %. In thiscase, the occurrence of miscellaneous crystals was suppressed, and thearea ratio of a melted portion 33 of the grown gallium nitride thin filmwith respect to the seed substrate was about 53%.

Example 3

Gallium nitride crystals were produced under two sets of conditions(production numbers 12 and 13) shown in Table 5 using the apparatus ofFIG. 1 in a manner similar to that of Embodiment 2 described above.Hereinafter, the production conditions will be described.

(Production Conditions)

Seed crystal: gallium nitride thin film substrate

Dimension: 14 mm×15 mm gallium nitride thin film (film thickness: 10 μm)

Gas species: nitrogen gas (N₂), purity 99.999%

Sodium: purity 99.9 to 99.99%

Gallium: purity 99.999 to 99.99999%

Hydrocarbon: Lamp oil, boiling point: 150° C. to 250° C.

Crucible used: Al₂O₃ (alumina), purity 99.9 to 99.99%

Direction in which seed crystal is placed: vertical

Growth time: 144 hours

Conditions, such as sodium, gallium, the type of a hydrocarbon thatcoats sodium, and an exhaust time and a heating time of the step ofremoving a portion of the hydrocarbon, are shown in Table 5 below. Theresults from these two sets of production conditions are shown in Table6. Confirmation of generation of gallium nitride, the amount of a growncrystal, the amount of miscellaneous crystals grown, and the yield ofgallium are measured by a method similar to that of Example 1.

TABLE 5 Heating Growth Exhaust temper- temper- Growth Production Na GaHydro- time ature ature pressure No. (g) (g) carbon (min) (° C.) (° C.)(MPa) 12 2.3 2.0 Lamp 120  25° C. 865 3.6 oil 13 2.3 2.0 Lamp 20 120° C.865 3.6 oil

TABLE 6 Crystal Miscellaneous Growth Produc- Confirmation growthcrystals GaN area tion of amount growth yield ratio No. generation (g)amount (g) (%) (%) 12 Confirmed 1.71 0.12 71 82 13 Confirmed 1.81 0.0275 92

Firstly, as shown in Table 6, a gallium nitride crystal was confirmedfor all production numbers 12 and 13. The amount of miscellaneouscrystals grown was 0.12 g or less for all production numbers 12 and 13,which was smaller than the amount of miscellaneous crystals grown whenno hydrocarbon was added in Example 1, so that the effect of suppressingnonuniform nucleation was confirmed. Also, since sodium was coated witha hydrocarbon (lamp oil), oxidation of sodium or the like was prevented,so that the resultant crystal had high quality.

FIG. 4 shows a photograph for production number 12. For productionnumber 12, sodium was coated with a lamp oil, and gas was removed at 25°C. for 120 minutes in an adjustment step. A gallium nitride crystal wassatisfactorily grown on substantially an entire surface of the galliumnitride thin film at a growth area ratio of 82%, though a portion of thegallium nitride thin film was melted (melted portion 33). For productionnumber 13 (not shown), sodium was coated with a lamp oil, and gas wasremoved at 120° C. for 20 minutes. In this case, the growth area was92%, i.e., more satisfactory growth.

Example 4

A gallium nitride crystal was produced using the apparatus of FIG. 5 bythe method of Embodiment 2 described above. The production conditionsare described below, and the results are shown in Table 7 below. Notethat, in Table 7, “With carbon corting” refers to this example and“Without carbon coating” refers to a comparative example.

(Production Conditions)

Seed crystal: gallium nitride thin film substrate

Dimension: 2-inch gallium nitride thin film (film thickness: 10 μm)

Gas species: nitrogen gas (N₂), purity 99.999%

Sodium: purity 99.9 to 99.99%

Gallium: purity 99.999 to 99.99999%

Hydrocarbons:

-   -   Lamp oil: boiling point 150° C. to 250° C. (for coating sodium)    -   Liquid paraffin, boiling point: 170° C. to 340° C. (for coating        sodium)

Crucible used: Al₂O₃ (alumina), purity 99.9 to 99.99%

Direction in which seed crystal is placed: fixed to bottom of crucible

Growth time: 144 hours

A reaction vessel 80, a crystal growth vessel (crucible) 82, gallium 86,and sodium 84 were placed in a glove box (not shown), followed bycharging of materials. Initially, a seed 88 was set in the crystalgrowth vessel 82. Next, the sodium 84 and the gallium 86 were weighedand then placed in the crucible 82. The sodium 84 was coated with ahydrocarbon (a lamp oil or a liquid paraffin) as described in Embodiment2 after removal of oxides or impurities on the surface, and was thenplaced in the crucible 82. Thereafter, the crucible 82 in which thematerials were set in the glove box was set in the reaction vessel 80.Also, in the glove box, a valve 65 at a gas inlet side and a valve 66 ata gas outlet side were closed so that the sodium 84 in the crucible wasnot oxidized even if the reaction vessel was removed into the air.Further, a high-pressure chamber lid 61 was opened, and the reactionvessel was set in the crystal growth apparatus via a joint 64.Thereafter, reaction gas (here, nitrogen gas) was caused to flow via agas flow rate adjuster 62. In this case, the valve 65 and the valve 66were opened to allow the nitrogen gas to flow into the reaction vessel.In this situation, the high-pressure chamber lid 61 was closed and thehigh-pressure chamber 60 was evacuated. When a predetermined degree ofvacuum was reached, the gas flow rate adjuster 62 was temporarily closedand the chamber 60 was evacuated to high vacuum. Thereafter, evacuationwas performed while causing the reaction gas to flow again, therebyremoving a desired amount of the hydrocarbon (a lamp oil, a liquidparaffin, the crucible in the reaction vessel.

Although a hydrocarbon, such as a lamp oil or the like, that isrelatively easily evaporated, can be removed by gas flow or evacuationeven at room temperature, a time required to evaporate the hydrocarboncan be effectively reduced by holding the hydrocarbon, for example, atabout 100 to 200° C. for about 30 min to 2 hr while causing nitrogen gasto flow. When the hydrocarbon is a liquid paraffin or the like, whichhas a high boiling point, the hydrocarbon may not be sufficientlyremoved at room temperature by evacuation while causing nitrogen gas toflow. In this case, the reaction vessel may be temporarily held at 200°C. to 300° C. (e.g., 1 to 2 hours), and the flow of nitrogen gas may befurther continued, thereby making it possible to adjust the amount ofthe hydrocarbon remaining in the crucible. Note that, when completely nohydrocarbon is used, the sodium 84 may react with oxygen or water due toan influence of a slight amount of a residual impurity or the like in apipe or the like remaining in the joint 64 and the valve 65.

After leaving an appropriate amount of the hydrocarbon, the crystalgrowth temperature and the growth pressure were adjusted intopredetermined values. Conditions for crystal growth used herein are agrowth temperature of 850 to 870° C. and a growth pressure of 3.4 to 3.8MPa.

As shown in FIG. 6( a), a crystal having a seed layer 102 of about 10 μmthat was vapor-grown on a 2-inch holding substrate 100 (here, sapphirewas used) was use as a seed.

TABLE 7 Film Presence of thickness miscellaneous (mm) crystalsColoration Yield Without 1 to 1.5 Yes Yes 2/5 hydrocarbon coating With 2to 2.5 Substantially Substantially 4/5 hydrocarbon no no coating

As shown in Table 7, under the conditions that a hydrocarbon was added,a grown crystal 104 having a thickness of about 2 to 2.5 mm that wasLPE-grown on a seed was able to be grown with high reproducibility. Onthe other hand, when no hydrocarbon was used for coating of sodium, theprobability (yield) of crystal growth was lower than when a hydrocarbonwas used, and the film thickness that was able to be achieved was asthin as 1 to 1.5 mm. This is a main reason why nonuniform nucleationoccurs (so-called occurrence of miscellaneous crystals) on a wall of thecrystal growth vessel other than the seed crystal even if growth iscarried out under the same conditions. Further, when a surface of sodiumwas coated with a hydrocarbon, a crystal with substantially nocoloration was able to be grown with high reproducibility. On the otherhand, when no hydrocarbon was used, a crystal with deep coloration wasfrequently observed. In some cases, a case where only a black thin filmcrystal was obtained was frequently observed.

As described above, when sodium was coated with a hydrocarbon, not onlymiscellaneous crystals occurred less, but also the yield of crystalgrowth was able to be improved and a highly transparent crystal was ableto be grown with high reproducibility. Moreover, when sodium is coatedwith a hydrocarbon, the amount of the hydrocarbon tends to be largerthan the optimal value of Example 1. Therefore, a portion of thehydrocarbon was removed by evaporation, so that a GaN crystal was ableto be grown on an entire surface of a 2-inch seed crystal withoutexcessive melting (excessive meltback) of the seed layer.

Next, the holding substrate 100 was removed by polishing, andthereafter, a growth surface of the LPE-grown crystal 104 (FIG. 6( b))was mechanically or chemical-mechanically polished, so that a 2-inchself-sustaining GaN substrate 106 (FIG. 6( c)) was able to be obtained.

Example 5

As Example 5, a case where a large number of self-sustaining GaNsubstrates are obtained using an LPE-grown substrate 106 as a seed willbe described with reference to FIG. 7. As shown in FIGS. 7( a) and 7(b),a GaN crystal 108 of about 4 mm was able to be grown on the seed crystal106 under the same growth conditions as those of Example 4, except thatthe growth time was two times longer than that of Example 4. Here, as inExample 4, by adding a hydrocarbon, growth was able to be carried outwith substantially no occurrence of miscellaneous crystals, even thoughthe growth time was as long as 288 hours.

A seed portion of the obtained crystal of FIG. 7( b) was removed. Thecrystal was cut into slices each having a thickness of about 1 mm usinga wire saw, followed by polishing of the rear and front surfacesthereof. Thereby, crystals 110 each having a thickness of 400 μm (FIG.7( c)) were able to be extracted from about 2-inch crystal. Thesecrystals were obtained from an LPE-grown crystal as a seed. Therefore, aGaN self-sustaining crystal substrate having excellent crystallinity, anEPD density of 1×10⁴ to 5×10⁵ (cm⁻²), and a low dislocation degree wasable to be obtained. Also, the transparency of the crystal wassatisfactory and the coloration of the crystal was substantially notobserved. In this case, the optical absorption coefficient with respectto a wavelength of 400 to 620 nm was able to be 10 cm⁻¹. Therefore, bycoating sodium with a hydrocarbon, the yield of crystal growth and thereproducibility of the transparency can be caused to be satisfactory inaddition to the effect of suppressing miscellaneous crystals by thehydrocarbon. The miscellaneous-crystals suppression effect was moreadvantageous, particularly when long-time growth is required for largethickness or bulk growth. Although a dopant was not added in Examples 1to 5, an n-type dopant (Si, O, Ge or Sn) or a p-type dopant (Mg, Sr, Baor Zn) may be added in an appropriate amount as described above.

INDUSTRIAL APPLICABILITY

According to the present invention, suppression of miscellaneouscrystals, an improvement in reproducibility, and a reduction incoloration can be carried out during the growth of a group-III elementnitride crystal using a flux made of an alkali metal.

1-12. (canceled)
 13. A group-III element nitride crystal comprising atleast one dopant selected from the group consisting of Si, O, Ge, Sn,Mg, Sr, Ea, Zn and Ca, the group-III element nitride crystal having anoptical absorption coefficient of 10 cm⁻¹ or less with respect to lighthaving a wavelength of 400 nm or more and 620 nm or less.
 14. Thegroup-III element nitride crystal according to claim 13, wherein thegroup-III element is at least one element selected from AI, Ga and In,and the group-III element nitride is a compound represented byAlsGatln_((1-s-t))N, where 0≦s≦1, 0≦t≦1, and s+t≦1.
 15. The group-IIIelement nitride crystal according to claim 14, wherein the group-IIIelement is Ga, and the group-III element nitride is GaN.
 16. A substratefor forming a semiconductor device including a group-III element nitridecrystal, wherein the group-III element nitride crystal is the group-IIIelement nitride crystal according to claim
 13. 17. A semiconductordevice, wherein a semiconductor layer is formed on a substrate, and thesubstrate is the substrate according to claim
 16. 18. The group-IIIelement nitride crystal according to claim 13, wherein content of thedopant is 1×10¹⁵ cm⁻³ or more.
 19. The group-III element nitride crystalaccording to claim 13, wherein content of the dopant is 1×10¹⁹ cm⁻³ orless.
 20. A group-III element nitride crystal, having an opticalabsorption coefficient of 10 cm⁻¹ or less with respect to light having awavelength of 400 nm or more and 620 nm or less, and having an EPDdensity of 5×10⁵ (cm⁻²) or less.
 21. The group-III element nitridecrystal according to claim 20, wherein the group-III element is at leastone element selected from AI, Ga and In, and the group-III elementnitride is a compound represented by AlsGatln_((1-s-t))N, where 0≦s≦1,0≦t≦1, and s+t≦1.
 22. The group-III element nitride crystal according toclaim 21, wherein the group-III element is Ga, and the group-III elementnitride is GaN.
 23. The group-III element nitride crystal according toclaim 20, having an EPD density of 1×10⁴ (cm⁻²) or more.
 24. A substratefor forming a semiconductor device comprising the group-III elementnitride crystal of claim
 20. 25. A semiconductor device comprising thesubstrate of claim 24 and a semiconductor layer formed on the substrate.